To meet governmental materials requirements for highway signs and/or signals, such as those usually encountered on the National Highway System (NHS), the signs and/or signals are required to meet certain performance characteristics. One of those performance characteristics is the ability to return or reflect light from a source back in the general direction of the source. Various patents exist that disclose materials or devices that provide this performance characteristic. Examples are disclosed in U.S. Pat. No. 106,764 (a kind of glass which transmits a fraction of its light and reflects the greater part), U.S. Pat. No. 235,913 (reflector, so arranged, intercepts the rising ray of light and reflects them angularly downward), U.S. Pat. No. 835,648 (triple reflector), U.S. Pat. No. 1,415,595 (light reflecting prism), U.S. Pat. No. 1,591,572 (central triple reflectors), U.S. Pat. No. 2,067,701 (double mirror), U.S. Pat. No. 2,167,149 (cube corner prism), U.S. Pat. No. 2,354,048 (reflexive reflection), and U.S. Pat. No. 2,432,896 (retro-reflective).
There are two types of commercially available retro-reflective materials used on the NHS; namely, laminated-sheet product and injection-molded plate made of glass or plastic. Both products require significant capital equipment to manufacture.
The retro-reflective laminated-sheet material is available in long length rolls usually 48″ wide. This product includes sheet or film mounted micro-reflective elements, such as glass beads or small, relative to sheet thickness, prismatic elements. This product is usually provided to sign manufacturers in rolls. The sign manufacturer unrolls and cuts the desired sheet length which is then glued to a substrate such as plywood or sheet aluminum. The Minnesota Mining and Manufacturing Company sells a similar product under the trademark SCOTCHLITE and includes minute glass spheres embedded in a matrix of synthetic resin to provide retro-reflection. The laminated-sheet material is not structurally self-supporting and is required to be adhered to a structurally self-supporting substrate. Typically, the laminated-sheet product, relative to the injection-molded plate product, has a short service-life due to progressive degradation of retro-reflective performance characteristics when exposed to NHS roadside environmental conditions. Examples of laminated-sheet products are disclosed in U.S. Pat. Nos. 2,310,790; 2,354,048; and 3,684,348.
The other type of retro-reflective material is injection molded plates having cube corner formations on one surface thereof, such molded plates being formed from glass or synthetic plastic. Cube corner reflectors molded from glass and more recently from acrylic resins have commonly been employed as safety devices on bicycles, automobiles and other vehicles. They are generally rigid, which do not lend themselves either to shaping to various substrates of nonplanar character. Making them thinner to make them more flexible would reduce the size of the cube corner formations, which makes criticality in control of angles and dimensions to be far more acute since even a minute deviation will produce a substantial deviation in the capability of the material to retro-reflect light rays impinging thereon. Examples of retro-reflective injection-molded plate products are disclosed in U.S. Pat. Nos. 1,800,673 and 2,205,638.
The retro-reflective injection-molded plate material is significantly smaller length/width dimensionally than the laminated sheet product. Typical sizes are 2″×3″, 4″×6″, 6″×12″ and 30″×30″. The bigger the size, the more expensive is the required mold and the attendant maintenance. The injection-molded plate, in application, is structurally self-supporting.
Both retro-reflective products have technological disadvantages for the governmental agencies which purchase a majority of the retro-reflective highway sign products in the United States. Four of the aforementioned disadvantages of the prior art are:
1) the sheet or film product's retro-reflective properties degrade relatively quickly, vis-à-vis the polycarbonate or acrylic injection molded product over time, due in part to its exposure to sunlight, due in part to its exposure to aggressive air content such as smog, and due in part to physical abrasions such as wind-laden sand or dirt, said degradation being highly site-specific. As such, government traffic engineers cannot predict in advance the level of retro-reflectivity a specific highway sign will have after a relatively short time of in-service. For example, if two otherwise identical highway signs using the sheet retro-reflective means, are placed next to each other, exposed to sunlight in the United States, with one facing north and the other facing south, all other factors remaining constant, the sign face oriented south will show significantly greater degradation over a short period of time then its identical companion sign which is oriented north. The degradation is, for the most part, due to exposure from UV radiation from sunlight.
2) one of the disadvantages of polycarbonate or acrylic cube-corner retro-reflective plates for highway signs is the present cost associated with highway sign sized injection molded parts. These signs are relatively large. For example, a standard stop sign is 36″×36″, an exit ramp sign 60″×60″ and overhead guide signs about 12-15′ long by 10-12′ high. Thus, the size requirement means that the prior art of manufacturing highway sign size cube-corner elements requires relatively expensive capital equipment making the individual sign, significantly more expensive than the sheet-faced highway sign. The one significant advantage that a polycarbonate or acrylic cube-corner retro-reflective highway sign has over the sheet-faced highway sign is that the polycarbonate cube-corner highway sign's retro-reflective properties, all the aforementioned factors remaining constant, degrade at a significantly slower rate.
3) both prior art means of providing retro-reflectivity to highway signs have the disadvantage of site-specific indeterminate retro-reflective degradation rates. In the case of sheet-faced highway signs, government traffic engineers frequently compensate for retro-reflective degradation by initially providing higher than required retro-reflective rated sheeting so as to provide a guessed at, site-specific, average retro-reflectivity over the service-life of the highway sign in question. In the case of polycarbonate or acrylic cube-corner signs, due primarily to the manufacturing's capital equipment costs, the selection of the level of retro-reflectivity offered is limited. That is, the government traffic engineer has very limited choices of levels of retro-reflectivity when the polycarbonate or acrylic cube-corner means is employed. While the polycarbonate or acrylic cube-corner degradation is much slower than the sheet-faced means, the traffic engineer is not able to tailor the site-specific retro-reflective need when using polycarbonate or acrylic cube-corner highway signs. Further, due to the expense of construction, polycarbonate cube-corner highway signs have been built using the polycarbonate or acrylic cube-corner material to provide long service-life retro-reflectivity and overlaying said cube-corner material with transparent and/or semi-transparent sheeting and/or films and/or inks to provide the sign's intended legend or message. Such sheetings, films and/or inks suffer the same quick degradation due to UV, abrasion, etc. and aforementioned.
4) both means of providing retro-reflective highway signs require that a vehicle's headlights illuminate the sign face before the retro-reflective properties can be availed for the benefit of the vehicle's driver. Three conflicting engineering design issues have recently conspired to complicate the government traffic engineer's highway sign selection and utilization due to the need for a vehicle's headlights to illuminate a sign's face. The three issues are:
A) a general need to elevate highway roadside signs such as STOP or YIELD signs, particularly in urban areas due to blocked line-of-sight for drivers who find themselves behind relatively larger vehicles such as delivery trucks, vans and the larger SUVs. That is, to allow a line-of-sight to a highway sign, during daylight hours, recent governmental traffic sign regulations have encouraged the placement of highway signs at higher elevations than that used in the past. By moving highway signs upward, the amount of retro-reflected headlight light and the retro-reflective angle of the reflected light, during nighttime hours is directly, adversely, affected. The design placement of highway signs commonly known as Guide Signs which are usually located on over-the-roadway overhead structures, also known as sign bridges, have also been encouraged to be placed at higher elevations so as to minimize the threat of being impacted by over-height vehicles has also resulted in reduced nighttime retro-reflectivity of sign faces.
B) The recent encouragement, by the U.S. Department of Transportation, for motor vehicle manufacturers to utilize European-style vehicle headlights which tend to project light forward and downward rather than the traditional American-style headlight which tended to cast light forward in a conic projection with the cone's axis, somewhat downward, but providing significant upward illumination, causes even the older highway signs located at lower elevations to be illuminated with far less headlight lighting thus providing less retro-reflected light off the sign face to the vehicle's driver.
C) The prior art of providing permanently fixed illuminating fixtures for highway signs now runs afoul of State and local government environmental laws for the protection of Dark Sky Conservation such as the State of New Mexico's and the State of Arizona's “Outdoor Lighting Control Act” and the City of Las Cruces “Lighting Control Act”.
Referring to FIG. 1, noted as “AA”, “AZ” & “AP” are the retro-reflective minimum values for the laminated sheets required by government agencies under the specific conditions noted, accounting for known degradation over time, Below the retro-reflective sheet minimum values, on FIG. 1, are the test results for test samples of multiple, large scale injection molded retro-reflective plates. The Illinois Department of Transportation (IDOT), Materials Test Lab results show, referring now to only the highest retro-reflective sheet material, that injection molded retro-reflective plates of similar area provide significantly higher retro-reflection. Specifically, referring to FIG. 1, “white” (also known as “crystal”) show injection molded plates at 1041 candellas per lux per sq. m. vs 640 candellas per lux per sq. m. requirements for sheet product.
It should be noted that the individual injection molded retro-reflective plates, making up the large, multi-plate, retro-reflective test plates tested by IDOT were of a geometry disclosed in U.S. Pat. No. 2,205,638 whereby one-half of the units with their maximum range of inclination towards the right and with the other half towards the left, assuming the borders to be vertical. The '638 patent devices, while less efficient in retro-reflecting light directed off center, produce very large retro-reflection observation angles. In fact, applicant observed good retro-reflection, during the IDOT testing, of as much as 80° off-axis and IDOT reported acceptable retro-reflection in excess of 80° off-axis. In sharp contrast, retro-reflective sheeting is usually at best no more than 30° off-axis and the angularity is subject to degradation.
In summary, the placement of highway traffic signing at higher elevations, the encouragement of the use of less-glare-producing European-style headlights for the benefit of on-coming motorists and with the evolving environmental laws restricting the self-illuminating of the signs has resulted in a reduced performance for the prior art of retro-reflective highway sign design, particular the very large green-and-white overhead guide-signs which are exclusively fabricated from retro-reflective sheet mounted on an aluminum structural substrate. Further, the prior art does not provide means and method of allowing the government traffic engineer to design and adjust the amount of retro-reflectivity desirable at a given site-location.
Accordingly, there is a need for an economic retro-reflective molded plate made in accordance with the present invention that will provide the economy of a substrate-mounted retro-reflective laminated-sheet but still exceed the initial retro-reflective laminated-sheet performance requirements and provide the utility of a retro-reflective injection-molded.
It should be understood that the term “sheet” means non-self-supporting, requiring a substrate, such as an aluminum plate, for structural support, and the term “plate” means structurally self-supporting material.